1. Field of the Invention:
The present invention relates to a container superior in crack resistance made up of an inner layer and an outer layer, said inner layer being made of a saponification product of ethylene-vinyl acetate copolymer (referred to as EVOH resin hereinafter) having a specific composition, melt viscosity index, and specific gravity, and said outer layer being made of a thermoplastic resin having a melt viscosity index in a specific range established according to the melt viscosity index of the EVOH resin.
2. Description of the Prior Art:
The conventional glass containers for agricultural chemicals and the conventional metal containers for gasoline, kerosene, and other petroleum products are being replaced by plastic containers, particularly those made of a thermoplastic resin like a polyolefin, especially polyethylene resin, because of its superior physical and chemical properties (e.g., light in weight, rust-resistant), sufficient mechanical strength, and impact resistance. Attempts are being made to improve plastic containers by providing them with an inner layer made of EVOH resin which is superior in oxygen barrier properties and resistance to oil and organic solvents, particularly hydrocarbon solvents. This idea, however, has not been put to practical use because of some problems yet to be solved. That is, the inner layer made of EVOH resin is liable to crack when it is kept in contact with a hydrocarbon solvent, polar organic solvent, or any chemicals containing such a solvent for a long period of time. Cracking occurs because EVOH resin is dried prior to molding and EVOH resin after molding (which is accomplished at about 200.degree. C.) is in the state of almost absolute dryness. This is a serious obstacle to putting EVOH resin to practical use.
Although much has been studied on the cracking of polyethylene and other resins, very little is known about why and how EVOH resins crack. Thus, so far there is no effective means available for preventing the inner layer of EVOH resin from cracking.
The cracking cannot be avoided by simply laminating an EVOH layer on a polyethylene layer as in the case of the container for agricultural chemicals made up of an outer layer of polyethylene containing calcium carbonate and an inner layer of EVOH resin, which is disclosed in Japanese Patent Laid-open No. 146183/1979.
In order to overcome the above-mentioned disadvantage, the present inventors carried out extensive studies and found that the cracking is associated with the degree of crystallization and the specific gravity of the EVOH resin involved. They also found that where EVOH resin is used in combination with a thermoplastic resin, cracking occurs frequently if the thermoplastic resin has a low melt viscosity index (abbreviated as MI value hereinafter), that the cracking of EVOH resin has something to do with the relationship between the MI value of EVOH resin and the MI value of the thermoplastic resin, and that cracking occurs more frequently in a container of composite structure than in a container made of EVOH resin alone.
The present inventors studied the cause and mechanism of cracking, the relation with property values of EVOH resin and other thermoplastic resin combined with EVOH resin, and the relation with the molding method. (It is assumed that residual stress resulting from molding promotes cracking in the containers of composite structure.) As the result of the studies, there was established a means for preventing cracking of EVOH resin used as an inner layer of containers of composite structure.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a multi-layered packaging container superior in crack resistance, said container being made up of an outer layer of thermoplastic resin and an inner layer of EVOH resin characterized in that the ethylene content is 25 to 60 mole %, the degree of saponification of the vinyl acetate component is higher than 96 mole %, the MI value is lower than 1, the specific gravity (d) is in the range specified by the formula (1) below, and Y.sub.1 and Y.sub.2 are in the range specified by the formula (2) below. EQU 1.274-4.4.times.10.sup.-3 X&lt;d&lt;1.332-4.4.times.10.sup.-3 X (1) EQU .vertline.Y.sub.1 -Y.sub.2 .vertline..ltoreq.1 (2)
where d is the specific gravity of EVOH resin at 25.degree. C., X is the ethylene content of EVOH resin, and Y.sub.1 and Y.sub.2 are the MI values of the thermoplastic resin and EVOH resin, respectively.
The MI value is measured at 190.degree. C. under a load of 2.16 kg in accordance with ASTM D 1238-65T, and it is expressed in terms of g/10 minutes.